The invention relates to interferometer arrangements, clock recovery devices, phase detectors, methods of detecting data transitions, methods of recovering clocks, and methods of comparing phases.
In any digital transmission system, it is necessary to recover the timing of the information being sent. Often it is considered inefficient to sent a separate clock signal, and so it is necessary to recover the timing from the data signal itself. In optical transmission systems this is usually done after conversion into electrical form, using one of two methods.
Firstly, a phase locked loop can be used to lock a separately generated clock to transitions in the data signal. Short periods where there are no transitions in the incoming data signal are bridged by the clock generator, which needs to be sufficiently stable.
Secondly, the incoming data can be filtered to extract a clock signal. In this case a resonant filter will be necessary, to fill in periods where there are no transitions. Furthermore, if the data is coded in non return to zero (NRZ) code for example, there may be substantially zero energy at the clock frequency. In these cases, a non-linear operation such as a squaring function is necessary to create a response, preferably a peak, at the clock frequency. A narrow band resonant filter can then be used to extract the clock frequency.
In optical transmission systems, such clock recovery arrangements have been used in regenerators and in receivers. However, electronic devices become expensive at high data rates, particularly when there is no other requirement for optical to electrical conversion.
There have been many attempts to implement more and more regenerator or receiver circuitry in optical form for improved performance or reduced cost. It is known to achieve a non linear function using an optical to electrical conversion device, then extract the clock frequency with a filter.
An early all optical regenerator is known from U.S. Pat. No. 5,446,573 using a non linear ring resonator comprising a semi conductor laser and phase modulators. However, it is difficult to make a practical device or integrate the arrangement.
Partly optical regenerators are known, where latching of the optical data input is carried out optically, following conventional electrical clock recovery. Optical clock recovery circuits were restricted to the use of mode-locked laser arrangements, eg as shown in U.S. Pat No. 5,548,433. As shown in FIG. 1, a coupler 1 is used to couple an input optical data signal to a laser 2. The phase and frequency of an output pulse stream is locked to the input, since the laser acts as a resonant narrow band filter. However, such methods are limited to use with optical data signals such as return to zero (RZ) coded signals which have sufficient energy at the clock frequency, unlike NRZ coded signals. Most high capacity optical transmission systems use NRZ coding.
Another arrangement which is limited to use with RZ coded signals is known from U.S. Pat. No. 5,574,588. An optical phase locked loop is created by detecting correlation between the input signal and a new clock signal by combining them and passing them through an optical amplifier. U.S. Pat. No. 5,504,610 shows achieving such an optical locked loop using an optical mixer to multiply two inputs. This correlation process requires that substantial energy be present in the data at the clock frequency.
One document which tries to address the limitation to RZ coded signals is U.S. Pat. No. 5,434,692, which discloses a device for use with NRZ, CMI (Code, Mark Inversion) and biphase coded data. First, a passive filter delay interferometer is used to linearly add the input data to itself. The fibre 10 needs to be lengthy in high bit rate systems, probably many kilometres, to avoid beat noise. A three level signal is produced, which drives an optical amplifier 11 for am to pm conversion.
This, together with a narrow band filter 12, give a non linear function which produces a response at the clock frequency. A resonant filter 13 can then extract the clock.
Such a device is not practicable for commercial data transmission systems because it is bit rate specific, impossible to integrate, and difficult to tune. The narrow band filtering element in particular would require piezo electric devices for tuning, which are insufficiently reliable for field use. Furthermore, for each different type of coding, the XOR logical operation would need to be changed.
It is an object of the invention to provide improved devices and methods.
According to a first aspect of the invention, there is provided an interferometer arrangement comprising an interferometer for receiving an optical signal modulated with data to vary an interference condition of the interferometer according to the data such that an output of the interferometer based on the interference condition is responsive to transitions in the data. By using an interferometer in this way, the timing of the transitions can be determined optically, without conversion to electrical form, independently of the type of coding used for the data. Furthermore, the arrangement can be produced in integrated form.
Advantageously, the interferometer is a two arm type. This type is easier to integrate, easier to control, and can handle a range of bit rates.
Advantageously, the arrangement comprises means for biasing the interferometer to give a peak output when the data is at an intermediate state between low and high states. This enables the response to the interferometer to be adjusted as desired.
Advantageously the interferometer arrangement is used in a clock recovery arrangement. This enables clock recovery independent of data coding without conversion of the data into electrical form.
Advantageously, the interferometer is arranged to receive the generated optical clock, and the interferometer output is dependent on the relative phases of the generated optical clock and the transitions in the optical data signal. This enables a phase lock loop to be constructed, in which the high bandwidth parts at least are implemented in optical form.
Advantageously, the clock recovery arrangement is used with an optical sampler to retime an optical signal. This enables a practical all optical regenerator to be created capable of handling different types of data coding. Optical retiming may also be useful at a transmitter end or receiver end of the optical path.
According to another aspect of the invention there is provided a clock recovery device for recovering a clock from an optical data signal, the device comprising:
a phase locked loop; wherein
the loop comprises an interferometer arrangement; and
the device is arranged to recover the clock optically from the optical data signal.
An interferometer based phase locked loop enables clock recovery to be achieved optically, and for a variety of data coding types.
Advantageously, the interferometer arrangement is responsive to the transactions in the data.
According to another aspect of the invention there is provided a phase detector for comparing the phase of modulation of two input signals comprising:
an optical comparator for receiving the two input signals in optical form, performing a non-linear operation on the signals, and outputting a signal dependent on the relative phase of the modulations of the two input signals.
Advantageously the phase detector comprises means for distinguishing a difference frequency of the comparator output, from other beat products of the modulations. The difference frequency contains the useful information on the two input signals.
Advantageously the distinguishing means comprises a low pass filter. This can be implemented relatively easily.
According to another aspect of the invention there is provided a clock recovery device comprising an optical pre-processing stage for processing an optical data signal; and
a resonant stage for outputting a clock in response to an output of the pre-processing stage; wherein the two stages are integrated together.
To recover a clock from a range of optical data signals, a resonant stage require some sort of pre-processing. The provision of an optical pre-processing stage which can be integrated with a resonant stage enables a practical optical clock recovery device to be achieved.
According to another aspect of the invention there is provided a method of detecting data transitions in an optical data signal using an interferometer arrangement comprising the steps of:
inputting the optical data signal to the interferometer arrangement to vary an interference condition of the arrangement; and
biasing the arrangement to output a signal responsive to the transitions in the data.
According to another aspect of the invention there is provided a method of recovering a clock from an optical signal modulated by coded data by performing a non-linear optical operation on the data so as to enable the clock to be recovered independently of the type of coding used for the data. Performing a non-linear optical operation on the data, such as those described above, enables clock recovery to be achieved without conversion to electrical form, and thus the hardware requirements can be simplified.
According to another aspect of the invention there is provided a method of recovering a clock from a modulated optical signal comprising the steps of:
using an interferometer to perform a non-linear operation on the modulation of the optical signal, and
filtering the output of the operation to recover the clock.
Using an interferometer to perform a non-linear operation enables a variety of data coding formats to be handled and enables integration, to reduce costs.
According to another aspect of the invention there is provided a method of recovering a clock from an optical data signal, comprising the steps of:
inputting the optical data signal to an interferometer comprising an optically active element; and
deriving a clock synchronised to the data signal.
Advantageously the interferometer comprises a two arm interferometer.
According to another aspect of the invention there is provided a method of comparing the phases of modulations of two input signals comprising the steps of:
inputting the signals in optical form to an optical comparator;
performing a non-linear operation on the signals; and
outputting a. signal dependent on the relative phase of the modulations.
Preferred features discussed above may be combined as appropriate, as would be apparent to a person skilled in the art. They may be combined with any aspect of the invention as appropriate.